The present invention relates to apparatus and methods for providing substrate structures having metallic layers for microelectronics devices such as, for example, chip-on-board packages, board-on-chip packages, dynamic random access memory packages, micro-ball grid array packages, and the like.
As the trend toward decreasing the size and increasing the density of microelectronics packages continues, surface mounted chip-on-board (COB) packages, ball grid array (BGA) packages, and chip-scale-packages (CSP""s) are commonly used to increase packaging density and reduce lead lengths for improving die performance. In such microelectronics packages, proper attachment of the die is a fundamental requirement.
FIG. 1 is a side cross-sectional view of a COB package 10 in accordance with the prior art. In this COB package, a die 12 has a bottom surface 14 attached to an electrically-insulative substrate layer 16, such as a printed circuit board, by an attachment layer 26. Typically the electrically-insulative substrate layer 16 is composed of bismaleimide triazine (BT), a polymer designed for printed circuit boards and the like, although other materials may be successfully used, including epoxy-based materials such as FR4 and FR5. The substrate layer 16 may have an internal network or mesh of interwoven fibers (not shown) to improve the strength and rigidity of the substrate layer 16.
The COB package 10 has a plurality of bond pads 18 formed on the die 12 and a plurality of contact pads 20 formed on the substrate layer 16. A lead wire 22 electrically couples each bond pad 18 to one of the contact pads 20 in the conventional manner. An encapsulating material 24 (or glob top) is formed over the die 12, the bond pads 18, the contact pads 20, and the lead wires 22 to hermetically seal and protect these sensitive components from mechanical stress, humidity, oxidation, and other harmful elements.
Successful die attachment involves proper and consistent alignment of the die to a packaging substrate for improved automatic bonding yield. The die attachment should desirably be uniform and void-free over the contact area between the bottom surface 14 and the attachment surface 28 to provide good mechanical strength and thermal conduction. The die attachment should also be free of flakes or other debris which may later come loose and cause a malfunction of the microelectronics package.
Typically, the die 12 may be attached to the substrate layer 16 by applying an adhesive material, such as an epoxy adhesive, onto a die attachment surface 28. The die 12 is then positioned over the epoxy adhesive and pressed against the epoxy adhesive to form a thin, uniform adhesive layer 26 between the bottom surface 14 of the die 12 and the die attachment surface. The adhesive layer 26 may then be cured, such as by heating the COB package 10 in an oven, to bond the die 12 to the substrate layer 16.
Alternately, the die 12 may be eutectically bonded to the substrate layer 16. Eutectic bonding takes place when two materials melt together (alloy) at a lower temperature than either of them separately. The two eutectic materials most commonly used for die attachment are gold and silicon. Although the melting point of gold is 1063xc2x0 C. and the melting point of silicon is 1415xc2x0 C., when the two materials are mixed together, they alloy at about 380xc2x0 C. Methods of eutectic die attachment are described, for example, in U.S. Pat. No. 5,037,778 issued to Stark and Whitcomb, and in U.S. Pat. No. 5,760,473 issued to Dickson and Max, which patents are incorporated herein by reference.
For eutectic die attachment, a layer of gold may be plated onto the die attachment surface 28. The COB package 10 may then be heated so that the gold layer alloys with the silicon bottom surface 14 of the die 12 to form the attachment layer 26. Alternately, a layer of alloy material composed of gold and silicon may be place on the attachment surface 28, and the COB package 10 heated so the gold and silicon layer alloys and bonds with the silicon bottom surface 14 and silicon attachment surface 28. With the die 12 located in the desired position, the die is compressed against the liquid gold-silicon alloy and moved in a xe2x80x9cscrubbingxe2x80x9d action to form the eutectic attachment layer 26. The COB package 10 is then cooled to complete the eutectic bond, thereby attaching the die 12 to the substrate layer 16.
Die attachment using an epoxy adhesive layer is favored over eutectic bonding for its economy and ease of processing. Epoxy adhesives, however, do not provide the strength of eutectic bonding, and may decompose at high temperatures, such as those experienced during bonding of the bond pads 18 and contact pads 20 and sealing of the COB package 10. Also, the attachment surface 28 of the substrate layer 16 may contain voids or surface irregularities that degrade die attachment, particularly those substrate layers having an interwoven mesh of fibers.
The present invention relates to apparatus and methods for providing substrate structures having metallic layers for microelectronics devices. In one aspect of the invention, an apparatus includes an electrically-insulative substrate layer, and a metallic layer attached to the electrically-insulative substrate layer, the metallic layer being attachable to a bottom surface of the microelectronics device. The metallic layer may advantageously provide a surface free from voids or irregularities for improved attachment of microelectronics devices. The metallic layer may also provide improved conduction of thermal energy away from the device, shielding from electromagnetic interference, a moisture barrier between the device and the substrate, and may serve as a convenient ground channel. In one aspect, the metallic layer may be continuous layer. Alternately, the metallic layer may be segmented into a plurality of closely-fitted pieces, or a plurality of spaced-apart pieces separated by expansion joints.
In another aspect, an apparatus may include a second metallic layer formed on the electrically-insulative substrate layer opposite from the first metallic layer. The second metallic layer may improve rigidity of the substrate layer and may provide additional shielding for the die from electromagnetic interference. Alternately, a solder resist layer may be formed on the first metallic layer to protect and mask the first metallic layer during processing. In a further aspect, a plating layer is formed on the second metallic layer.
In yet another aspect, a microelectronics package includes an electrically-insulative substrate layer, a metallic layer attached to the electrically-insulative substrate surface, an attachment layer formed on at least part of the metallic layer, and a die having a bottom surface attached to the attachment layer. The attachment layer may be an adhesive layer, or alternately, a eutectic layer. In another aspect, a microelectronics package may include a second metallic layer attached to the electrically-insulative substrate layer substantially opposite from the first metallic layer. In still another aspect, a plating lever may be disposed on the second metallic layer.